Autopilot



Dec. 5, 1961 H. K. SKRAMSTAD ETAL AUTOPILOT Filed Jan. 17, 1952 7 Sheets-Sheet 1 1 l2 Q l3 9LU i/ GAIN SWITGH l4 VH5 FINAL REFERENCE REFERENC E l V V \\\\I MISSILE RELEASES 20 Q /6 IL ygaa GAIN 8| ALTITUDE REFERENCE CONTROL ALTIMETER ALTITUDE REFERENCE INVENTORS H. K. SKRAMSTAD CHARLES RAIDENH/SH JOHN A. HART BY 1% 4M)? ATTORNEYS AUTOPILOT '7 Sheets-Sheet 2 Filed Jan. 17, 1952 INVENTORS H. K. SKRAMSTAD ll llllllb CHARLES RAUDE/VBUSH .uomv A. HART fl Al I ll I EEE I EE I lllll ll 1 Dec. 5, 1961 H. K. SKRAMSTAD EIAL 3,011,738

AUTOPILOT 7 Sheets-Sheet 3 Filed Jan. 17, 1952 mvl-zu'rons H. K. .SKRAMS 7140 CHARLES RAUDEIVBUSH JOHN A. HART ATTORNEYS AUTOPILOT '7 Sheets-Sheet 4 Dec. 5, 1961 H. K. SKRAMSTAD ETAL Filed Jan. 17, 1952 Dec. 5, 1961 H. K. SKRAMSTAD ETAL AUTOPILOT 7 Sheets-Sheet 5 Filed Jan. 17, 1952 INVENTORS H. k. SKRAMSTAD CHARLES RAUDENH/SH JOHN A. HART BY %M(9/'M Dec. 5, 1961 H. K. SKRAMSTAD ETAL 3,011,738

AUTOPILOT Filed Jan. 17, 1952 '7 Sheets-Sheet 6 300 v REG +300 V REG.

\ INVENTORS H. K. SKRAMSTAD CHARLES RAUDEIVBUSH JOHN A. HART ATTORNEYS FIG. .9

Dec. 5, 1961 H. K. SKRAMSTAD ETAL 3,011,738

AUTOPILOT Filed Jan. 1'7, 1952 '7 Sheets-Sheet 7 i\ 5/ ll SUMMATION AMPLIFIER LATE RAL CONTR INVENTORS H. K. smausmo GHARLES muomausu JOHN A. HA I 3,011,738 AUTOPILUT Harold K. Skramstad, Charles Raudenbush, and John A.

Hart, Riverside, Calih, assignors to the United States of America as represented by the Secretary of the Navy Filed .lan. 17, 1952, Ser. No. 266,978 10 Claims. (Cl. 244-77) (Granted under Title 35, ELS. (lode (1952), see. 266) This device relates to an autopilot for controlling the flight of aircraft, and more particularly to an autopilot for directing and stabilizing the airflight path of a guided missile, in accordance with received information.

The particular type of guided missile with which the preferred form of autopilot is to be illustrated is one utilizing two moveable flight control surfaces, such as elevons, in the wings in conjunction with fixed vertical and horizontal stabilizers in the tail, and for further information relating to the characteristics of this missile reference is made to the application of William H. A. Boyd, Serial No. 655,538, filed March 19, 1946 for Aircraft, now abandoned,, and that of Dundas P. Tucker, Serial No. 204,057, filed January 2, 1951 for Method of Attack on Marine Targets and Missile Therefor, and that of William H. A. Boyd, Serial No. 200,680, filed December 13, 1950 for Guided Missile and Assembly Thereof, now Patent No. 2,992,794.

This autopilot is adapted to control the missile heading while in flight in the azimuth and elevation directions, in response to two electrical command signals, and to stabilize the missile attitude in pitch and roll in accordance with a gyroscopically established reference'standard. The command signal directing the autopilot response in the azimuth direction may be derived from a missile radar and control system, such as disclosed in the application of Frederick C. Alpers, Fred S. Atchison and Wilfred A. Yates, Serial No. 296,772, filed July 1, 1952 for Control System for Homing Guided Missile, and the command signal directing the missile heading in elevation may be derived from an altimeter of any appropriate design within the missile. To facilitate an understanding of the autopilot operation in response to the above-mentioned command signals, a brief description of a particular missile attack path or flight program for pursuit navigation is here given, being the flight program which the subsequently ntent f described specific embodiment of the present invention is I designed to effect. This missile is carried by a mother craft, which releases it in the air at any altitude up to approximately 5000 feet and at any range up to approximately 20 miles, from a selected target. After the missile is released, it seeks the selected target by homing on it in azimuth, while seeking a low level altitude, elevation attack path. This low altitude at which the missile is to travel while homing in azimuth on the target is predetermined and will hereinafter be called the final altitude reference. As the rapidity with which the missile in seeking the final reference attack path altitude from its release altitude is limited by considerations 'of missile aerody namic stability, the characteristics of the azimuth homing system, and permissible autopilot construction, a steep dive or steep climb to this reference altitude by the misisle after release would not always prove satisfactory; and accordingly a compromise is made permitting the missile to reach the reference altitude in a finite time while allow ing for the above-mentioned missile characteristics. However, it is to be understood that other altitude programs may be selected, keeping in mind the basic end results, that the missile after release is vto home on the target in azimuth during its approach toward the target along a low altitude level attack path,

Although the above-mentionedmissile azimuth attack ice path utilizing pursuit navigation has been found to be satisfactory, there are instances when a modification of this mode of attack is desirable. For example, if the missile is launched at a relatively long range from a fast moving target, pursuit attack results in a curved azimuth homing path resulting in an unnecessarily long missile flight. These and other undesired effects may in some measure be compensated by introducing lead navigation, and accordingly provision is made in the preferred autopilot to be illustrated hereinafter, for introducing an additional command signal to the autopilot directing the missile to lead the selected target in azimuth. This additional signal may be derived from the above-mentioned It is accordingly one object of this invention to provide an autopilot for controlling the flight heading of an aircraft.

A further object of this invention is to provide an autopilot for controlling the flight heading of a high speed aircraft having moveable elevons and fixed stabilizer controllers.

i A further object of this invention is to provide an autopilot for controlling the flight path of a high speed aircraft in azimuth and elevation simultaneously by differential and unison movement respectively of two controller members on the aircraft.

A further object of this invention is to provide an auto.- pilot for an aircraft whichsimultaneously controls the path of this aircraft in azimuth and elevation while stabilizing the aircraft attitude along the desired path.

A further object of this invention is to provide an autopilot for guiding the path of an aircraft in azimuth toward a given target in response to an electrical command signal while simultaneously controlling the aircraft along a predetermined elevation path.

' A further object of this invention is to provide an autopilot for guidinga homing missile toward a moving surface target along a predetermined elevation path.

A further object of this invention is to provide an autopilot for controlling and stabilizing the flight path of an aircraft in elevation and azimuth simultaneously in response to two electrical command signals.

A further object of this invention is to provide an autopilot for controlling the elevation heading of 'ahigh speed aircraft in response to the altitude of the aircraft above a preselected reference. I v

A further object of this invention is to provide an autopilot for controlling the elevationheading of a high speed aircraft in response to the altitude of the aircraft above a preselected altitude reference and responsive to a second preselected altitude for vary'ing'the sensitivity of the elevation control.

A further object of this invention is to provide an autopilot for controllingthe elevation heading of a high speed aircraft in two modes in responseto the altitude of the aircraft above or below a preselected altitude point.

A further object of this invention is to provide an autopilot for controlling the azimuthheading of a high the present invention, taken in connection with the accompanying drawings forming a part of this specification and in which:

, FIG. 1 is an illustrative diagram of a selected-altitude attack path of the missile from its release point in the .air to the point at which it disgorges an explosive payload toward a surface target;

FIG. 2 is a block diagram of the overall autopilot circuits;

FIG. 3 is a detailed schematic diagram of one embodiment of the summation amplifier circuits;

FIGS. 4 and 5 are preferred voltage limiting circuits employed with the various summation amplifier circuits of FIG. 3;

FIG. 6 is a functional diagram of a preferred embodiment of altitude reference compensator circuit;

FIG. 7 is a detailed schematic diagram of the preferred altitude reference compensator circuit of FIG. 6;

FIG. 8 is a detailed schematic diagram of a preferred type of servo amplifier;

FIG. 9 is a detailed schematic diagram of a second embodiment of the summation amplifier circuits; and

FIG. 10 is a block diagram representation of a second embodiment of the control summation amplifier circuit employing the circuit of FIG. 9.

Referring now to FIG. 1 showing an altitude program of the missile from the point at which it is released from the mother craft until striking the target, the missile is designated by the numeral 10, the mother craft by numeral 9, and the target by numeral 20. If it be assumed the missile is released at the maximum above-mentioned range and altitude, an altitude path substantially resembling that designated as 12 is preferably followed. After release at the relatively high altitude of point 8, an electrical signal is generated by an altimeter mounted within the missile. This signal is compared with a fixed reference, representative of a low altitude reference (point 16) and a signal representing the error between the missile altitude and this first reference altitude is sent to the missile autopilot which directs the missile to reduce this error to zero by seeking this reference altitude. For the illustration shown the missile is commanded to dive. Although at high release altitudes, as here shown, it is desired that the missile quickly seek the final reference attack altitude of point 15, allowances for missile aerodynamic stability, and for azimuth homing maneuvers are made, and a maximum dive angle of from the horizontal is imposed upon the craft. The missile dives at this maximum angle until the altitude of point 13 is reached. At this much lower altitude, approximately 1500 feet for this specific embodimerit, the autopilot in response to the difference of the altimeter and first reference signals directs the missile to gradually pull out of its 15 dive in a uniform manner. For example at altitudes of approximately 1400 feet the missile dives at a 14 angle, at 1300 feet at a 13 angle etc. During this pull out the missile reaches an altitude represented by point 14. By this time the craft has completed most of its drop from the high release altitude and is relatively closeto the final reference attack altitude of point 15, and also to the water surface, and it is now desired that the missile quickly reach the final reference altitude and stabilize about this altitude. Accordingly, at the altitude of point 14 additional autopilot circuits (gain switch) respond to the altimeter signal and bring about two operations. In the first, the low altitude reference signal with which the altimeter signal is compared to derive the altitude error, is changed from altitude reference 1 corresponding to the altitude of point 16 to the final reference corresponding to the higher altitude of point 15. This first operation is carried out to prevent the missile in its dive from altitude point 14- from overshooting its former reference 1 and diving into the water. In the second operation at point 14, the autopilot circuits are made more sensitive to the altitude and final reference difference error signal permitting the missile to quickly reach the final reference altitude of point 15 and stabilize about this altitude, as may be seen from FIG. 1. After the missile reaches altitude point 14, and the above-mentioned effects take place, the maximum dive or climb angles allowed the missile in response to altimeter and new reference error signals are still limited to allow for missile azimuth maneuvers while homing on the target, but the increased sensitivity provided permits a response of the missile to altitude correction commands approximately ten times as great as that taking place during the altitude program prior to the gain switch being thrown at altitude point 14. After the missile has reached the final reference attack altitude point 15, it stabilizes about this reference altitude While continually homing on the target in azimuth as before, and when approaching the target and a given predetermined range therefrom is reached, it releases an explosive charge or payload, such as a sonic homing torpedo (shown at point 17) which thereupon enters the water and directs itself at the target 29 by its own propulsion and its own homing guidance, as is well-known to the art.

The illustrated altitude path 12 of FIG. 1 is not intended as a scale representation of the distances traveled by the missile from the point of release 8, until releasing the sonic homing torpedo 17, for assuming the missile is released at a 20 mile range from the target, the major portion of this 20 mile attack path occurs at the low level final reference altitude of point 15, shown by the broken line representation. Therefore, in FIG. 1 the path from altitude point 3 to altitude point 16 is purposely expanded in relative length for purposes of showing the various phases of the preferred dive program when the missile is release at the maximum allowable altitude.

Referring now to FIG. 2 showing in block diagram form the preferred overall autopilot circuits, two command signals directing the autopilot circuits to control the missile, one from the altimeter (not shown) and the other from the missile radar (not shown), are sent in on lines 21 and 44 respectively. The autopilot circuits receiving these command signals, as may be seen from FIG. 2, may be generally broken down into two groups A and B. Each of these groups of circuits are shown enclosed by a dotted line, group A being responsive to the altimeter signal from line 21, hereinafter to be called the Elevation Channel of the autopilot, and group B being responsive to the azimuth command signal from line 44 hereinafter to be referred to as the Azimuth Channel of the autopilot. Each of the above-mentioned autopilot channels comprises in essence a servo slaving arrangement to compel the missile heading to follow the command signals. This is, the Elevation Channel A circuits slave the missile elevation heading to the altimeter to minimize the above-mentioned altitude error, and the Azimuth Channel B circuits slave the missile azimuth heading to the missile radar to minimize the radar azimuth error signal. Each of these channels are responsive independently to their respective command signals, but their outputs cooperate and act simultaneously in effecting the desired missile heading providing for unison movements of the missile elevons and differential movements of the clevons to bring about changes in elevation and azimuth heading respectively.

In the Elevation Channel designated A, the altimeter signal indicative of missile altiude is sent in over line 21 to an altitude reference compensator 22. The altitude reference compensator operates in three distinct modes in response to the magnitude of received signals. For all large altimeter signals representative of missile altitudes above point 13 of FIG. 1, this device generates a fixed signal representative of altitude reference 1 (point 16 of FIG. 1) and subtracts this fixed signal from the altimeter signal to generate the above-mentioned altitude error, while limiting the generated altitude error signals for all altitudes above point 13 of FIG. 1 to the magnitude of error signal generated at point 13. In the second mode taking place for missile altitudes between points 13 and 14, the fixed generated signal representative of altitude reference 1 is subtracted from the altimeter signal to generate a true altitude error, and in the third mode taking place for all altimeter signals representative ofmissile altitudes below point 14 of FIG. 1, this device generates a fixed signal representative of the final reference altitude.

(point 15 of FIG. 1) and subtracts this fixed signal from the altimeter signal to yield an altitude error, signal, and amplifies this error si nal (more correctly eliminating attenuation present in the first mode). In the second and third modes no limitin is imposed on the altimeter error signal by the altitude reference compensator circuit, leaving the output signal of compensator 22 directly responsive to the deviation of missile altitude from the first and final reference altitudes respectively, however, the error signal in the third mode is amplified to provide increased sensitivity of the autopilot to the error signal.

The altitude error signal is then conducted to the elevation summation amplifier 24 over line 23 where the error signal is amplified, and further limited 48 for allowing stability and azimuth control of the missile as will be more completely comprehended from subsequent discussions, and the resulting signal output of 48 is then sent to the longitudinal control summation amplifier 32 and limiter 63 for further voltage amplification. The twice voltage amplified and limited altitude err-or signal is then sent out over lines 33 and 34 in opposite phase, as ShOWn, to the port and starboard servo amplifiers, where the input signals are power amplified to drive the port and starboard actuators 37 and 56 respectively. These servo amplifiers are of the differential type wherein application of like polarity signals to corresponding terminals, and application of opposite polarity signals to opposite terminals bring about energization of both elevon actuators in the same direction, while reversing the polarity in either of the above instances energizes the elevon actuators in opposing directions. Hence as shown in FIG. 2 the opposite polarity signals from the output of the longitudinal control summation amplifier 32 and limiter s3, energizing the port and starboard servo amplifiers 35 and 54 respectively at opposite terminals, brings about like polarity signals on lines 36 and 55 which energize the port and starboard actuators, driving the port and starboard elevons respectively, so that these elevons are moved in unison and in like direction to bring about changes in the missile elevation heading. Of course, as the Elevation Channel A neglecting the limiting devices comprises pure linear amplification, the amount of this unison elevon movement is directly related to the altitude error comprising the difference between the altimeter signal and the particular reference signal employed.

Movement of the elevons 39 and'58 causes to be ge erated on lines 28 and 2'9 respectively, feedback voltages in opposition to the commanding signal on lines 27 feeding the longitudinal control summation amplifier 32. These voltages are generated by two energized potentiometers 61 and 60 respectively, whose variable center taps are mechanically ganged to elevons 39 and 58 respectively, such that if it be assumed that movement of both elevons 39 and 53 upward moves center taps of potentiometers 61 and 60 to the left, and downward to the right, it is seen that signals of like polarity are generated on lines 28 and 29 by unison movement of the elevons in either direction, while two signals of opposite polarity, canceling each other, are produced on these lines by equal. differential or opposite direction elevon movements. For the unison elevon movement as brought about by the Elevation Channel A of the autopilot in response to the altitude error, these feedback voltages close the servo loop causing it to act as a position slaving device, i.e. the elevons move in like direction until these feedback signals balance the commanding signal on line 27 and then stop; But as mentioned above like movement of the elevons cause the missile elevation heading to change and as the missile elevation heading changes a signal is generated by a vertical gyro 40 located within the missile structure, which functions as a standard to generate a signal over line 25 whose magnitude indicates the amount of missile pitch, and whose polarity indicates the direction of missile, pitch from the horizontal. Thus, as the missile responds in elevation heading to the altitude error from the altitude reference compensator, a signal is generated from the vertical gyro in opposition to the altitude error command, telling the autopilot channels that the missile is responding to the command by diminishing the signal on line 27 to the longitudinal control summation amplifier, such that when the missile has assumed the elevation heading angle called for by the error signal, a zero signal overline 27 to the control amplifier 32, directs the elevons to return to their zero position. Thus in effect the altitude error calls for change in elevation heading and drives the elevons to cause the missile direction to vary, until the gyro signals that the missile has assumed the new elevation angle called for, at which time they are returned to zero position and the missile continues along this dive or climb angle until a change in altitude error takes place which drives the elevons to position the missile until its vertical gyro indicates a compliance with the new command.

Autopilot azimuth control As discussed above, during this altitude program the missileis simultaneously directed in azimuth to home on the selected target, and signals from the missile radar are continuously sent in over line 44 to the Azimuth Channel B of the autopilot. These signals are conducted to the azimuth signal summation amplifier 47 where they are amplified, and then to limiter 49 where the magnitude of the amplified signal is regulated. The output limited signal is then conducted to the lateral control summation amplifier 51 over line 50 where further voltage amplification takes place, and the twice amplified signal is then sent in like phase to opposite terminals of the port and starboard servo amplifiers'35 and 54 respectively, over lines 52 and 53. It will be recalled that this manner of Channel A, causes diiferential power amplified signals over lines 36 and 55 to drive the port and starboard elevon actuators in opposite directions and hence cause differential or opposite port and starboardelevon displacements. As the port and starboard elevons 39 and 58 respectively are differentially displaced, two feedback signals of like phase and of magnitude dependent upon the amount of differential displacement are sent back over lines 42 and 43 to the lateral control summation amplifier 51 in the Azimuth Channel in opposition to the amplifier and limiter azimuth command signal on line 5%. These feedback signals are taken from the moveable taps of energized potentiometers 61 and 62 which are mechanically ganged to port and starboard elevons 39 and 58 respectively as in the Elevation Channel. Here it is seen that differential movements of the elevons (i.e. upward movement of elevon 39 moving tap of potentiometer 61 to the left producing a more positive signal on line 42,

downward movement of elevon 58 moving tap of posignal is sent out by additional means within the abovementioned ventical gyro standard over line 41 whose magnitude is indicative of the amount ofroll from the reference attitude and whose polarity indicates the dire'ction (clockwise or counterclockwise) of roll from the reference. Thissignal is sent to the lateral control sum-- mationamplifier 51 in opposition to the azimuth 'command coming in over line 5t) resulting in a diminishing signal to the elevon actuators as the missile responds to the azimuth command. When the missile rolls to the extent called for by. the command inazimuth the difierential displacement of the elevons is reduced to zero. As the. azimuth command varies'the elevons are again displaced I until the gyro roll signal indicates the responded to the command.

missile has p As noted above, and in the introductory material, the azimuth command signal entering the Azimuth Channel B of the autopilot over line 44, is derived from the missile radar and control system, and this command directs the autopilot to compel the missile to home by pursuit on the target in azimuth. Assuming that lead navigation is now desired, this azimuth command is integrated by devices within the radar and control system and introduced as an additional command to the Azimuth Channel B. This additional azimuth lead command is introduced into the azimuth signal summation amplifier 47 input through a summing resistor over line 46, as shown in FIG. 2, and compels the Azimuth Channel B of the autopilot to direct the missile in azimuth to lead the selected target in accordance with the magnitude of this integrated signal.

It should be noted that the feedback from the elevon otentiometers 61, 62, and 6% due to their polarity of energization act independently for the two autopilot channels (Elevation and Azimuth). Unison displacement of the elevons in either direction feeds back equal adding or like polarity signals only to the Elevation Channel A, and canceling signals, having no elfect, to the Azimuth Channel B, while differential displacement of the elevons in either direction feeds back equal adding or like polarity signals only to the Azimuth Channel B, and canceling signals to the Elevation Channel A.

Summarizing, both autopilot channels, the Elevation Channel A and the Azimuth Channel B, simultaneously activate the port and starboard servo amplifiers which control the two elevons of the missile. The lateral control summation amplifier 51 of the Azimuth Channel tends to drive the elevons in opposite directions producing a roll and resultant change in azimuth, and the Elevation Channel A of the autopilot drives the servo amplifiers so as to direct the elevons to move in the same direction causing a change in missile elevation heading and resultant change in altitude. The longitudinal control summation amplifier 32 has a limiter 63 such that elevon commanding signals from the Elevation Channel leave a reserve for azimuth maneuvers, while the lateral control summation amplifier 56 has no limiting device controlling the output of the Azimuth Channel thereby allowing autopilot azimuth control to take precedence over elevation control when large commanding signals of equal value are simultaneously introduced in both channels. This type of control is desirable to prevent the missile radar from losing sight of the selected target and insuring a homing flight to the target.

Stabilization The above discussion shows in detail the manner in which the autopilot channels control the elevation and azimuth heading of the missile by comparing the present missile heading in elevation and position in roll, as indicated by the vertical gyro, with the desired missile flight direction as indicated by the altimeter error command and homing azimuth command, and thereafter simultaneously positions the elevons to correct the missile heading to these commands. However, other factors may affect the missile heading, tending in some instances to divert its elevation and azimuth direction from the path prescribed by the directing commands. Some of these factors may comprise outside forces such as winds, and other atmospheric conditions, and others may be aerodynamic unbalance of the missile body, or variation of the missile center of gravity caused by' diminishing weight of the propelling fuel. Thus, a compensating or stabilizing system is desired to correct for these forces tending to aerodynamically unbalance the missile, or to correct for forces merely tending to divert the missile from its azimuth homing path and preselected altitude program. This stabilization function is performed in part by the vertical gyro located within the missile body and electrically connected in the follow-up paths of the Elevation and Azimuth Channels.

As the vertical gyro.

responds to any missile elevation heading differing from a preset standard or any missile roll differing from a preset standard by generating electrical signals proportional thereto, any undesired missile pitch or roll causes electrical signals indicating these undesired changes to be generated by the vertical gyro in the feedback paths of the autopilot Elevation and Azimuth Channels. These signals are compared with the command signals indicating desired missile heading and a correction is made by positioning the elevons to overcome this pitch or roll and return the missile heading to the desired position.

In addition to this vertical gyro compensation for instability, additional jaw stabilizing means are provided by a gyro stabilized antenna and control system, a complete disclosure of which is found in the related application of Perry R. Stout et al., Serial No. 219,106, filed April 3, 1951 for An Object Tracking Antenna and System of Missile Guidance.

Lead and lag networks in all positioning servo or slaving systems a certain amount of compensation is necessary, in some instances for speeding up response, and in others slowing it down to provide the desired response and prevent oscillation. The present device provides for elevon feedback voltages that are not only indicative of elevon position but that also anticipate elevon movement. This anticipation function has been introduced to prevent elevon oscillation, and as may be seen by reference to FIG. 2 is introduced in the elevon feedback paths 28, 29, '42, and 43 for both the Elevation and Azimuth Channels taking the form of parallel resistor condenser lead circuits 31, 30, 30a, 3312, respectively. An additional lead circuit 31:: is interposed in the gyro roll signal line 41 feeding the lateral control summation amplifier 51 in the Azimuth Channel B where it has been found to aid in roll stability.

Two time delaying networks in the form of T-lag net- Works are designated 26 and 45 in FIG. 2. These networks are interposed in the pitch input line 25 from the vertical gyro 40, and in the azimuth input line 44 from the azimuth signal source respectively, to delay the autopilot elevation and azimuth response to instantaneous error signal changes. As the altimeter response to altitude is relatively gradual as compared with the vertical gyro pitch signal over line 25, this lag network 26 improves pitch stability, and at the altitude point 14 (FIG. 1) when altitude error sensitivity is effected by approximately a 10 to 1 change, this lag prevents pitch oscillation. The network 27 in the Azimuth Channel B is supplied to prevent a roll oscillation with large azimuth commands, due to the missile inertia.

In the illustrated system, network 26 was supplied with components bringing about a 4/ 10 second lag which effectively prevented oscillation at altitude point 14 for a change of l to 10 in altitude error sensitivity.

Detailed circuitry Before entering into a detailed disclosure of the preferred circuitry employed to perform the'above-mentioned functions, it is well to note that for each of the circuits of FIG. 2 in block form, any arrangement of components may be substituted providing they perform the functions above enumerated. For example, each of the signal summation amplifiers 24, 32, 47, and 51, along With limiting means 48, 63, and 4% respectively, may be in the form of any type of analogue adding circuit which sums, amplifies, and limits the input signals. The servo amplifiersSS and 54 may be any type of differential input power amplifier with double ended output, the elevon actuators polarity responsive mechanical positioning devices,

etc.

Signal summation amplifiers (FIG. 3)

v FIG. 3 is a detailed schematic circuit diagram of one preferred embodiment of the longitudinal control summation amplifier 32 of FIG. 2. The general circuit configuration of the remaining summation amplifiers embodied in the autopilot, including the elevation signal summation amplifier 24, the azimuth signal summation amplifier 47, and the lateral control summation amplifier 51 are preferably similar to the longitudinal summation amplifier, however the individual gains, amplification balancing means, and limiting means may differ in the several circuits in accordance with their respective functions as will be pointed out hereinafter. Each of these units in the preferred embodiment include four cascaded stages of direct current amplification, the first three stages of which comprise identical plate coupled differential amplifiers, and the fourth stage a differential cathode follower. Two feedback paths are provided from the cathode circuit of the fourth stage to the input grid and cathode circuit of the first stage for permitting the overall amplifier characteristics to be modified in accordance with the requirements for summation amplifiers, and for gain and stability purposes, as is conventional in the art. Referring now to FIG. 3 a dotted line generally designated 81 encloses the first stage comprising two halves of a double triode, type ZCSl, designated 71 and 72, connected back to back through equal cathode resistors of 1K ohms designated 79. The plate circuit of each tube is energized from a common power supply voltage at a positive value of 270 volts through a 470K resistance. Equal one megohm (1M) coupling resistors connect each of the electron tube plates to the opposite terminals of a balancing circuit, generally designated 82, comprising a parallel circuit of three series connected resistors of 2M (fixed value), 5M (variable), and 2M (fixed) shunted by a capacitor of .2 ,ufd. A minus power supply voltage of 270 volts is applied to the variable rtap of the .5M resistance in the balancing circuit, and is also supplied to the junction of the equal valued cathode resistors 79 through a feedback resistor 80 of 680K ohms. Connecting the grid of tube 72 to ground is a high value resistance of 100K ohms, and the grid of tube 71 is energized by the summed input voltages from 27, 3t and 31 as may be seen from FIG. 2. With no signal on the grid of tube 71, the relatively high value of plate load resistance in each tube'circuit, and the high value of feedback resistance 80 connecting the cathode resistors 79 to the minus supply and furnishing the current retun path, substantially determines the amount of current flow through each tube, whose direction is indicated by the arrows. This current flow, due to the stage component connection symmetry, is the same through both tubes, and a combined flow equal to twice this value is conducted through the common feedback resistor 80 to the 270 voltages minus supply. As the current flowing through each tube is initially the same, the voltage drops across the tube plate load resistors (470K) are equal and points 81 coupled to these equal potential points through the above-mentioned 1 megohm coupling resistors conduct equal value voltages to the grids of succeeding tubes 73 and '74 in the second stage.

When a voltage energizes the grid of tube 71, the resulting change in tube impedance varies the current flow from the 270 volt source through the 470K plate resistance, tube 71, cathode resistor 79, and common feedback resistor 80 to the -270 volt return. This change of current through feedback resistor 80 varies the voltage drop there across, changing the grid cathode potential of companion tube 72, to quickly permit the current flow through tube 72 to vary an equal and opposite amount resulting in the original voltage drop across feedback resistor 8i As variations of current flow through tubes 71 and 72 cause similar changes in their plate potentials, equal and opposite voltage variations appear at points 81 energizing the grids of the second stage tubes 73 and 74. Hence, it is seen that the difference in voltage between points 81, comprising the output of the first stage, is a direct function of the energizing input voltage. Each of the succeeding second and third stages operates in a posed by the Azimuth Channel B of the autopilot.

similar dififeren'tial manner, tubes 74 complementary to 73, and tube 76 complementary to 75, such that the difference of the two voltages energizing tubes 77 and 78 respectively of the fourth stage is a direct amplified function of the input. The fourth stage includes two symmetrically arranged tubes 77 and 78 generally connected to operate as a difierential cathode follower output stage, wherein these tubes are connected back to back through equal cathode resistors of 27K ohms, which are shunted by a balancing circuit comprising two equal series connected resistors of 36K ohms Whose series junction point is grounded. The plates of each tube are connected together and commonly energized by the 270 volt direct current source through a relatively low resistance of 12K ohms, and the differential voltage output from the third stage including the complementary electron tubes 75 and 76 energizes the grids of these tubes. Output voltages from the fourth stage, as is common to the cathode follower construction, is taken from the cathodes as shown in FIG. 3 over lines 33 and 34. A resistor of 560K ohms, shown as enclosed within a dotted line generally designated 63, is connected to the grids of tubes 77 and 78 in shunt with the two series connected output coupling resistors of 2.2M supplied by the third stage. The value of this resistor is much lower than the combined values of the output coupling resistors that it shunts in the fourth stage grid circuit, and therefore presents a relatively low resistance circuit in the input of the cathode follower stage. a As the coupling resistors of the third stage, of value 1M, in series with the grid input resistors (2.2M) to the fourth stage act as a potential divider, this relatively low value of shunting resistor 63 serves to attenuate the third stage output, and effectively aid in varying the value at whichthe circuit limits as will be pointed out hereinafter.

This differential type of amplifier stage construction provides a means wherein a D.-C. signal may be stably amplified Without danger of change of output signal due to variation of supply voltage source, or as termed by the art, without drift. In the above device, as seen, the output signal, comprising the difference between the voltages on output lines 33 and 34, is a direct function of the summed input signals, and assuming a change in power supply voltage this output signal remains constant. For example, an increase in the 270 voltage positive supply not only'produces a like change in the plate potentials of tubes 7]., 73, 75, and 77, but also produces a similar increase in the plate potentials of complementary tubes 72, 74, 76, and 78 respectively, as all these tubes are energized from the common 270 volt D.-C. source. As the output voltage comprises in essence the difierence between the plate voltages of the complementary tubes, this increase is canceled effecting no change in the output difference signal. In the illustrated embodiment, comprising the longitudinal summation amplifier 32, it may be observed from the overall block diagram of FIG. 2 that the double ended output signals on lines 33 and 34 are utilized to drive in like direction the port and starboard elevons respectively, and hence it is desired these two signals be balanced with respect to ground, that is, not only their difierence be independent of drift but also their magnitudes with respect to ground distance than the starboard elevon with a given elevation input command signal. This resulting diiferential elevon displacement produces a undesired tendency of the missile to roll, slip and turn in accordance with the altitude error signal which obviously conflicts with the control im- This unbalance is initially compensated by the actionfof the balancing circuit 82 above referred to in the output circuit of the first stage within dotted enclosure 81, wherein a variation of the moveable tap of the .SM potentiometer permits magnitude adjustment of differential energizing signals on lines 81 to the grid of the second stage tubes 73 and 74.

Limiting circuits for the signal summation amplifiers (FIGS. 3, 4, and

In the detailed disclosure relating to the preferred illustrative embodiment of the longitudinal summation amplifier 32 of FIG. 3, reference was made to the vary- .ing limiting requirements of the remaining summation amplifiers within the autopilot circuits. These amplifiers depending upon their location in the Elevation Channel A, or the Azimuth Channel B, and their positions in these channels may be required to impose a limit upon the maximum value of signal transmitted to their succeeding circuits, and also to provide in addition to the direct limiting function a measure of adjustable signal balancing.

Referring again to FIG. 3, the limiting function is supplied by the final cathode follower output stage comprising tubes 77 and 78. There, it may be seen that when .a sufficiently large positive grid signal is received by tube 77, a current flows from the plate circuit of proceeding tube 75 through the one megohm coupling resistor to the grid of tube 77 and subsequently to the cathode circuit of tube 77. This grid current flow causes a voltage drop across the relatively high one megohm coupling resistor, which as is well known to the art limits the maximum voltage that may be transmitted by output line 33. However as all of the summation amplifier-s are substantially identical devices this limiting function would normally occur for the same maximum value of input voltage energizing each, which as pointed out above is undesirable due to the varying limiting requirements of the several amplifiers. Hence, limit varying resistors, such as the one shown by FIG. 3 enclosed within a dotted line designated 63, are placed in shunt with the series connected 2.2M coupling resistors in the output of the third differential amplifier stage. These limit varying resistors depending upon their value with respect to the shunted coupling resistors serve to attenuate the voltage reaching the grids of the fourth stage differentially arranged cathode followers, varying the steady state, or no signal grid bias applied to these tubes. As the value of grid cathode voltage reaching the final stage tubes sufiicient to cause grid current flow remains fixed, this variation of steady state bias varies the maximum signal that may be applied to the first stage input before this maximum grid cathode voltage suiiicient to cause grid current flow in the final stage is reached, and hence varies the value at which the overall amplifies limits.

FIG. 4 illustrates a preferred form of limit varying and balancing circuit for application in the elevation signal summation amplifier 24, shown as enclosed within a dotted line designated 48, to conform with the similarly numbered block representation of FIG. 2, and comprises two fixed equal valued resistors of 180K ohms in series with a variable potentiometer of 250K ohms having a grounded center tap. The ends of this series circuit connection are applied to the respective grids in the final stage of the above-mentioned elevation signal summation amplifier 24 in a similar manner to the connection of limit varying resistor 63 in the amplifier circuit of FIG. 3. As the value of the series connected resistors is approximately the same as that of resistor 63 of FIG. 3, the limiting action provided by the elevation signal surnmation amplifier 24 approximates that provided by the longitudinal control summation "amplifier 3-2, however, the addition of the grounded variable tap of the 250K potentiometer provides a means for controlling the relative voltage with respect to ground, applied to the grids of the complementary tubes in the final stage as may be desired, thereby providing additional balancing.

FIG. 5 illustrates similar limit varying and balancing means for application with the azimuth signal summation amplifier 47. As shown, the value of the series resistance connection is greater than that of either FIG. 3 or FIG. 4, and hence this circuit limits at a lower value the output voltage of this amplifier, however the center tap of the 250K potentiometer is grounded through a 10K fixed resistance providing a more insensitive balancing action than the directly grounded limit varying circuit 48 of FIG. 4.

F imctional altitude reference compensator (FIG. 6)

FIG. 6 is a diagram for functionally representing the operation of a preferred embodiment of the altitude reference compensator circuit 22 of FIG. 2, the preferred detailed schematic of which is shown in FIG. 7 and will be described hereinafter. This circuit as briefly referred to above in connection with the overall autopilot block diagram of FIG. 2, is located in the Elevation Channel designated A, and is adapted to electrically respond to the altimeter command signal 21, and generate an altitude error signal 23 to the remaining autopilot elevation circuits for controlling the missile pitch. More specifically, this device generates an error signal proportional to the difference between the instant missile altitude, and a fixed altitude reference point, which signal energizes the elevation signal summation amplifier 24 and the remaining elevation autopilot circuits to direct the missile to dive or climb until the reference altitude is reached as indicated by a zero error signal. In order to permit the autopilot circuits to direct the missile along a selected altitude attack path, such as the path illustrated by FIG. 1, this device provides for two distinct modes of operation depending upon missile altitude above and below a selected point, which for the instant illustrative altitude program constitutes altitude point 14, shown in FIG. 1 and termed the gain switch altitude. In the first mode of operation, taking place above the gain switch altitude 14, the device compares the instant missile altitude with a selected fixed low altitude reference, shown for this illustration as point 16 of FIG. 1 and termed altitude reference 1, and generates an altitude error signal to the succeeding circuits indicative of the difference of these values, however in the event that the missile is released at extremely high altitudes, such as altitudes above pullout point 13 of FIG. 1, the altitude signal is limited to its value at pull-out point 13 before the above-mentioned comparison, in order to prevent unduly large error signals from energizing the autopilot and compelling the missile to dive too steeply. In the second mode taking place below the gain switch altitude 14, the altimeter signal is compared with a new fixed altitude reference, corresponding to the slightly higher altitude point 15, and termed the final reference point, and an error difference signal is generated. This error signal, due to the increased autopilot sensitivity desired at this time, when the missile has completed most of its dive from the illustrated high altitude release point, is amplified by the device permitting the Elevation Channel of the autopilot to rapidly direct the missile to reach the final reference altitude and stabilize about this altitude, through the remainder of the missile attack path.

Referring now to FIG. 6, the altimeter designated 96 generates a positive signal, indicative of missile altitude, over line 21 through three series connected resistors 165, 165, and 107 to the input line 23 of a signal summation amplifier 24. Connecting the junction of resistors 1G5 and 1% to ground is a voltage magnitude clipper enclosed within a dotted line generally designated 1G4, and comprising in the simplified illustration a diode electron tube in series with a fixed positive voltage to ground. A shorting switch 103, shown in FIG. 6 as open, is interposed across series connected resistors and 1%. Two fixed negative voltage sources 98 and 13 a 99 are alternatively connectable, through a single pole double throw switch 101, in series with three resistors 108, 109, and 110 to the common input line 23 of the above-mentioned signal summation amplifier 24, the positive terminals of these sources being grounded. Across series connected resistors 108 and 109 is interposed a shorting switch 102 similar to switch 103, whose contacts are open. A gain and altitude reference circuit, shown in block form designated 97, comprises a switching device electrically responsive to all values of the altimeter signal below a given level and operable by means of a mechanical linkage, shown by dotted line 100 to simultaneously close switch 103, shorting out series resistors 105 and 106; close switch 102 shorting out series connected resistors 108 and 109; and reverse switch 1011 replacing fixed negative altitude reference source 99 by negative reference source 98 in the series circuit comprising resistors 108, 109, and 110 to summation amplifier 24. Assuming the missile is released at a high altitude such as point 8 of FIG. 1, the circuit operates in accordance with its first mode as follows; the altimeter 96 generates a positive signal over line 21, indicative of this altitude, through resistor 105. This signal for all altitudes above point 13 is limited by the action of the voltage clipper circuit 104, which as is well known to the art may be adapted to limit signals above any desired magnitude to their magnitude at the selected level, while having no effect upon voltages below the selected magnitude. Thus altimeter signals generated above altitude point 13 are limited to the value corresponding to altitude point 13, while those generated at altitude 13 or below are unaffected by the clipper circuit. All these altimeter signals, whether limited or not, then pass through resistors 106 and 107 to the input 23 of signal summation amplifier 24. The negative voltage 99, corresponding to the altitude ref erence 1, point 16 of FIG. 1, conducts a negative signal through series summing resistors 108, 109, and 110 to the input 23 of the signal summation amplifier 24. The positive signal coming from the altimeter 97, and the negative signal from the altitude reference 1, 99 are added by the action of the summation amplifier 24 and what may be called the summing resistors 105, 106, 107, 108, 109, and 110 respectively, such that the resulting potential on line 23, constituting the input to the summation amplifier 24 and termed the altitude error signal is proportional to the difference between the missile altitude as seen by the altimeter and the reference 1 altitude. As noted above, the gain and altitude reference circuit 97 is responsive only to altimeter signals below a given voltage level, which for the instant embodiment constitutes the altitude level of point 14 of FIG. 1, and hence during the first mode of operation of the circuit of FIG. 6 switches 101, 102, and 103 remain in the positions shown permitting the altitude error signal at input line 23 to vary through the range corresponding to the altitude differences shown in FIG. 1 between points 8 and 1d, and between points 14 and 16. When the missile during its dive reaches point 14, termed the gain switch altitude, this circuit enters upon its second mode of operation. The gain and altitude reference circuit 97 responds to the altimeter signal and actuates mechanical linkage 100 to reverse switch 101 replacing altitude reference source 09 by a new and more negative reference source 08 corresponding to the final reference altitude point 15 of FIG. 1; closes switch 102 shorting out resistors 108 and109;j and closes switch 103 shorting out resistors 105 and 106. This operation changes the value of the altitude error signal entering input line 23, to the remaining autopilot circuits, providing a comparison between the instant missile altitude and a new and higher reference altitude corresponding to point 15 of FIG. 1,, Whichtherefore directs the missile autopilotto reduce this error to z 'ero by bringing the missile to this new altitude reference and i4 stabilizing it about this new reference 2. Shorting out the corresponding summing resistors 105, 106, 108, and 109 permits a greater percentage of the altimeter signal and the new altitude reference signal to reach amplifier 24 due to the smaller values of summing resistors now in circuit, comprising 107 and respectively. This increases the difference error signal reaching summation amplifier 24, and hence effectively increases the gain and sensitivity of the following circuits to this signal difference. As pointed out above, amplification is desirable at this time when the missile has completed the greater portion of its dive and the diiferences between the altimeter and reference signals are small, in order to permit the missile to quickly and accurately stabilize about this attack altitude.

Altitude reference compensator (FIG. 7)

FIG. 7 is a detailed schematic diagram of a preferred embodiment of the altitude reference compensator 22, comprising a plurality of selected circuits for performing the operations described in the functional representation of FIG. 6, wherein these selected circuits are enclosed by dotted lines designated to conform to the simplified block circuit representation there shown. Referring now to FIG. 7, the gain and altitude reference control circuit enclosed within a dotted line generally designated 17 comprises an electronic switching device responsive to two signals, the altimeter signal conducted over line 111, and a fixed negative signal corresponding to the gain switch altitude of point 14 of FIG. 1 conducted over line 117. This device operates to compare these signals, and switch two relay contacts 103, and in-opposite positions depending upon the polarity of the resulting input signal. More specifically when the altimeter signal is more positive than the gain switch altitude reference signal is negative, the resulting input signal is positive, and the relay contacts are in the positions shown in the FIG. 1, however when the magnitude of the altimeter signals drops below that of the gain switch reference signal, the resulting input signal is negative and both relay contacts are switched to their opposite positions. It is desired that this device respond accurately in performing this switching operation, and be substantially independent of power supply drift, and accordingly the device comprises in the preferred form two cascaded stages of differential D.-C. amplification similar to those used in the signal summation amplifier circuit of FIG. 3, feeding a push-pull power amplifier having two relays 119 and comprising the output load. Tracing a resultant positive input signal on the grid of tube 118, it may be seen that relay 119 is energized to the extent of pulling the contact arm of switch 103 to the left, and that relay 120 is deenergized so that contact arm of switch 115, is to the right. In a similar manner a resulting negative input signal deenergizes relay 119 causing its contact arm to move to the right, while energizing relay 120 thereby pulling its contact arm to theleft. These latter relay contact positions bring about "two effects in the remaining circuit, relay contacts 103 short out two resistors 105 and 106, and relay contacts 115 connect the junction point of resistors 109 and 110 to line 98;

The circuit enclosed by a dotted line generally designated 116 includes a stabilized negative potential divider comprising a negative source of 270' volts energizing a series connection of resistors of values 10K,

10K,'4K, 270 ohm, 360 ohm, and 2K to ground. Across the junction of the two l0K resistors to ground is shunted a voltage regulator tube of type 0B2, tending to-maintain a fiized voltage across the shunted resistors. The junction of the 2K and 360 ohm resistors, labeled 09' supplies a fixed negative potential, corresponding to altitude reference 1 (point 16 of FIG 1) to series connected summing resistors 108, 109, and 110 to output line 23. Similarly, the junction of the 360 ohm and 270 ohm resistors supplies a more negative fixed potential, corresponding to the final reference altitude (point 15 of FIG. 1) to line 98. A still more negative potential, corresponding to the gain switch altitude (point 14 of FIG. 1) is taken from the junction of the 270 ohm and 14K resistors and conducted to the input of the gain and altitude reference control circuit 97 over line 117.

The circuit within dotted line generally designated 104 comprises a diode electron tube of type 6AL5 connected in series with a positive stabilized voltage source to ground. This circuit is connected from the junction of summing resistors 105 and 106 to ground and serves to limit the positive value of the voltage at this junction in accordance with the value of the above-mentioned stabilized voltage as is well known to the art.

In operation the overall combination of circuits of FIG. 7, constituting the altitude reference compensator circuit 22, performs in the same manner as the above described functional representation of FIG. 6. In the first mode, taking place when the missile is at an altitude above that of the selected gain switch altitude point 14 of FIG. 1, the altimeter signal 21 is conducted through summing resistors 105, 106, and 107 to the output line 23. However, for all missile altitudes above pull-out point 13, the clipper circuit limits this positive signal to a value representative of altitude point 13. A negative signal from the altitude reference voltage divider 116 corresponding to altitude reference 1 (point 16 in FIG. 1) is taken from point 99 on the resistance voltage divider 116 and conducted through resistors 103, 109, and 110 to output line 23. The summing resistors 105-110, inclusive, in combination with the negative feedback resistor of the subsequent summation amplifier in effect adds these signals yielding an altitude error signal corresponding to the difference between the altimeter reading and the reference. During this first mode of operation the altimeter signal over line 111 is greater than the negative gain switch signal over 117 such that the resulting input energizing voltage to the gain and altitude control circuit 97 is positive and output relay switches 103 and 115 are in the positions shown. When the missile in its dive passes the gain switch'altitude point 14 of FIG. 1, the altimeter signal on line 111 drops below that of the negative signal on line 117 wherein a resulting negative input voltage to the gain and altitude control circuit 97 reverses relay switches 103 and 115, and the circuit enters its second mode of operation. Relay switch 103 shorts out summing resistors 105 and 106, and relay switch 115 connects the junction of resistors and 110 to line 98 of the altitude reference voltage divider 116. This latter connection effectively shorts out summing resistors 108 and 109 by shunting them across a low resistance of 360 ohms, and further connects summing resistor 110 to the lower negative voltage level of line 93 corresponding to the final altitude reference (point of FIG. 1). Hence during the second mode of operation the altimeter signal over line 21 is sent through only the single summing resistor 107 to output line 23, and a new reference signal corresponding to the final altitude reference is conducted through only the single summing resistor 110 to output line 23, thereby reducing the attenuation provided by the summation system and efiectively amplifying the resulting altitude error signal comprising the difierence between the altitude and the final altitude reference.

Servo amplifiers (FIG. 8)

The servo amplifiers including the port servo amplifier 35 and starboard servo amplifier 54 are identical circuits, and an embodiment of one (port) is shown in FIG. 8. These circuits, as generally enumerated above, are adapted to differentially amplify two signals applied to their input, yielding an output to drive elevon actuators whose magnitude and polarity is dependent upon the relative polarity and magnitude of the two input signals,

The schematic diagram of one preferred embodiment of the port servo amplifier 35 showing the detailed circuitry and values is enclosed within a dotted line designated 35 to conform with the block designation of FIG. 2. Within the dotted enclosure is a two stage D.-C. amplifier, the first stage connected as a two tube differential amplifier responsive to two input signals over lines 33 and 53 and adapted to transmit a push-pull output over lines 133 and 134, and the second stage a push-pull amplifier responsive to the signals over lines 133 and 134 and adapted to actuate the port elevon actuator 37 (FIG. 2) represented as two elevon actuating coils 137 and 138. The two tubes 126 and 127 of the first stage, which may be duo-triodes as shown, have their plates energized by two positive voltage sources of 270 volts through equal resistors of relatively high value. Connecting their cathodes to a common junction point 140 are two equal low-value resistors 131 and 132, and a high value feedback resistor is interposed between this junction point and a minus voltage supply of 270 volts furnishing a current return path for each tube through the common high value resistor 130. Plate coupling is provided by four series connected resistors all of high value and connected as two symmetrically arranged voltage dividers each shunting its related tube and the common cathode resistor 130. These resistors of values 820K, 1.8M, 1.8M, and 820K ohms are series connected in the above-mentioned order between the plates of tubes 126 and 127 respectively; and the junction of the two 1.8M ohm resistor is connected to the current return source of minus 270 volts. A transient by-pass capacitor 139 of value .01 ufd. is connected to shunt the values or" the series connected 1.8M ohm plate coupling resistors, across which terminal two diiferential output signals, on lines 133 and 134, are derived. Input voltages are conducted to the grids of tubes 126 and 127 over lines 33 and 53 respectively, which are shunted by two series connected grid leak resistors of 1M ohms, whose junction point is rounded. The high values of plate resistors 135, and 136, and the high value of common cathode feedback resistor 130, determines the steady state current flow through both tubes 126 and 127, and due to the symmetry of these tubes and associated components equal current flows through each. Energizing the grid of either tube over lines 33 or 53 varies the current flow therethrough, tending to change the voltage drop across the common feedback resistor 130. This relatively instantaneous voltage change is reflected as a negative feedback voltage to the grid circuit of. the other tube, thereby varying its current flow by an equal and opposite amount and maintaining the voltage drop across common cathode resistor 130 in its original steady state value. These respective tube current changes result in equal and opposite plate voltage changes for both tubes, which are transmitted by the above-mentioned plate and coupling resistors to the grids of tubes 128 and 129 over lines 133 and 134 respectively. For example, when a more positive signal energizes the grid of tube 126 over line 33, the current flow from the 270 volt source through plate resistor 135, tube 126, cathode resistor 132, and feedback resistor 130 to the minus 270 volt supply, is increased. The voltage drop acrossfeedback resistor 130 tends to increase, which generates a negative signal between grid and ground to tube 127"decreasing the current flow therethrough an equal and opposite amount. The increase in current flow through tube 126 lowers its plate voltage, while the decrease in current flow through tube 127 raises its plate voltage, and hence the output voltage on line 133 is lowered over steady state value while that over line 134 is raised over steady state value. In a similar'manner a negative signal energizing the grid tube 126 over line 33 .results in a more positive output voltage over output line 133, anda more negativeoutput voltage over line 134; and, as the circuit is symmetrically arranged energization of the grid of tube 127 negatively over line 53 results in line 134 increasing its positive voltage, and line 133 decreasing positively, while energization of input line 53 positively bringing about the reverse voltage changes. Thusthe differential output voltages on lines 133 and 134 have magnitudes and respective polarities dependent upon which of the input lines 33 or 53 is energized, or if both are energized upon which line receives the more positive potential. These differential output voltages are then conducted to the grids of tubes 128 and 129 constituting the second stage of the illustrative embodiment of the port servo amplifier. These tubes are connected as a conventional push-pull amplifier functioning to power amplify the differential input signals, and energize in push-pull array the port elevon actuators shown in simplified illustration as two coils 137, and 138 enclosed within dotted line 37. The current flows through the second stage in two paths; the first from a 75 volt positive source through actuator coil 137, tube 128, and resistor 140 to ground, and the second through the positive source of 75 volts, through actuator coil 138, tube 129, and resistor 140 to ground. With equal grid voltage signals, such as at steady state conditions, equal value currents flow through each of coils 137 and 138, and assuming these coils are differentially wound such that coil 137 exerts an upward pull on the port elevon, and coil 138 exerts downward pull on the port elevon, the forces exerted by these coils on the port elevon cancel whereupon it remains stationary.

However, differential energizing signals on input lines 133 and 134 vary the current flow through each of the two above-mentioned parallel paths, including the coils 137 and 138, by equal and opposite amounts, wherein an unbalanced force is exerted on the port elevon and it is positioned in accordance with this unbalanced force.

Utilizing this preferred type circuit, power supply drift, prevalent and troublesome in all direct curent amplification systems is eliminated, for, as in the summation amplifier circuit illustrated in FIG. 3, the above circuit responds primarily to diiference signals whereby variation of power supply voltages affect each tube by an equal amount leaving the difierence voltage unaffected.

Alternative summation amplifier (FIG. 9)

As briefly discussed above in the introductory material relating to the block diagram representation of the overall autopilot operation, the individual circuits shown in FIGS. 3-8 inclusive may be modified providing they perform in accordance with the functional requirements there set forth, and accordingly FIGS. 9 and 10 illustrate'alternative circuit embodiments for the summation amplifier circuits; FIG. 9 illustrating an alternative azimuth signal summation amplifier circuit 47, and its associated limiter circuit 49, and FIG. 10 an alternative lateralcontrol summation amplifier circuit 51. The alternative summation amplifier circuit of FIG. 9 comprisesa simpler device for performing the summation and amplification functions than the first illustrated embodiment of FIG. 3. This simplification in part results from eliminating the dilfer- 'ential type amplifier stages, 'andin partfrom the reduction in the number of amplifier stages. Elimination of differential amplification of the type shown by FIG. 3, is enabled due to the provision of a regulated power supply voltage for energizing the stage electron tubes thereby" providing substantially constant supply voltages that do not drift, wherein drift compensation formerly provided by the differential amplifier stages of FIG. 3 is no longer needed. Additional simplification provided by a reduction in the number of stages within-the summation amplifier, in this alternative" embodiment, is enabled due to higher gain as will 'theparticular amplifiercharacteristicsprovided by ina series resistance potential divider connected from the' regulated power supply voltage of 300 volts to ground and comprising a fixed resistor of K, in series with a variable potentiometer 149 of 25K, and fixed resistor 148 of 470K. A positive regulated voltage source of 300 volts energizes the tube plate through a resistance of 560K, and connecting the junction of this resistor and the plate to a regulated minus supply voltage source of 300 volts is a potential divider coupling network comprising three series connected resistors of 680K, 1M, and 1.2M. Across the output of this potential divider comprising the junction point of the 680K and 1M resistors, the plate signal is conducted to the grid of the second stage tube 154 of type 5670 through a high resistance 150 of 1.2M forming part of the signal limiting means. A transient by-pass capacitor of .02 afd. is also connected from the above-mentioned output of the first stage plate potential divider to ground. Tube 154 of the second stage is plate energized directly from the common regulated positive voltage source of 300 volts, and its cathode energized from the common regulated negative voltage source of 300 volts through an 85K resistance. Connecting the plate of this second stage tube to ground and also energized by the common regulated positive supply source voltage is a potential divider comprising two series resistors 152 and 153 of 270K and 5.5K respectively, whose junction point is connected to the grid of the second stage tube 154 by means of limiter diode connected tube 151 which may as shown comprise the second half of tube 154. The cathode follower output signal to ground is derived from the cathode of second stage tube across a 4.5K resistance, and this signal is conducted to subsequent circuits through resistor 50 of K. A feedback line designated 156 also conducts this output signal to the control grid of the first stage pentode 146 through a 2M resistance 145 constituting a negative feedback, and

conducts this output signal to the cathode of the first j'back line 156, and a small negative signal on thegrid of input tube 146, results-in a relatively high'positive' signal on output line 50 and feedback line 156. However, 'sharp limiting is imposed upon the magnitude of output signal of either polarity by the action of the limiting circuit, comprising high value resistor 150, diode connected electron tube 151 and resistor 153, for large values of positive signals reaching the control grid of output tube 154; and the output tube current cutoff provides the sharp limiting action for all large values of negative signals reach'ing the control grid of output tube 154. Qlt has been found that the provision of positive feedback in a D.-C. amplifier of thisnature allows the overall amplifiercharacteristics relatingoutput to input signals to approximate a vertical line, that is a smallinput signal resulting in a substantially instantaneous jump of theoutputsignal j to afhigh value as determined by the limiting devices, 1 thereby approximatingthe operation of an infinite gain amplifier. The additionof'a high value negative feed- "the input summing resistors 45 and'157 allows this device to sum the input signals, and amplifythesummed signals inaccordance with the ratio cff negative feedback resistor to the summing resistors as is well known to the art. Initially with no input energizing voltages, it is desired that the output voltage through 180K resistor 50 be low approximating volts, and thereby indieating to the subsequent autopilot circuits the no command condition, however due to the pure resistance coupling provided in the direct current type amplifier any variation in the value of circuit components such as change in the tube impedances, or change in value of the circuit resistors caused by heating effects or aging, may vary this so-called balanced condition or approximate 0 output voltage with no signal, and hence in order to correct for any such undesired effects, a variable balancing resistor 149 of 25K is provided in the screen grid circuit of first stage tube 146. The provision of this variable potentiometer allows manual adjustment of the screen grid energizing voltage over a given range permitting the output voltage of the first stage to be manually varied as desired in order to correct for, and balance out the unwanted effects.

Alternative control summation amplifier (FIG. 10)

The summation amplifiers throughout the autopilot, including the elevation signal summation amplifier 24, and the longitudinal control summation amplifier 32 in the Elevation Channel A; and the azimuth signal summation amplifier 47 and the lateral control summation amplifier 51 in the Azimuth Channel B, may each employ the differential amplifier circuit of FIG. 3 with only minor modification of component values, or limiting and balance means as discussed above. This versatility of the FIG. 3 amplifier embodiment permitting its application in all four units is enabled due to the inherent provision of a double ended output, which is taken from the cathodes of the differentially arranged cathode follower output circuit. However if it is desired that the four summation amplifiers each employ the second more simplified amplifier circuit of FIG. 9, modification of this circuit for providing a double ended output is needed. For example, three of the summation amplifiers as shown in the block diagram of FIG. 2 may employ single ended devices, and in these units the amplifier circuit of FIG. 9 may be used by merely modifying the component values, and limiting (clipper) circuit as desired, whereas the fourth is required to differentially drive the opposite terminals of the servo amplifiers by means of a double ended output and hence the single ended output circuit of FIG. 9 is not satisfactory. Accordingly FIG. 10 is an illustration of a control summation amplifier embodying the simplified circuit of FIG. 9 in conjunction with additional modifying means for converting the single ended output to a double ended output. Although this illustration relates to the lateral control summation amplifier 51, a consideration of the FIG. 2 block diagram shows that either one or both of the control summation amplifiers may employ double ended output signals for driving the port and starboard servo amplifiers.

Referring now to FIG. 10, the lateral control sum-mation amplifier block representation within a triangular enclosure designated -1 to conform the representation of FIG. 2, is energized by a plurality of inputsignals through summing lead networks 30a, 30b, and 31a, and through summing resistor 50. Double ended output lines represented as positive and negative are designated 169 and 52 respectively. Within the enclosure are two cascaded summation amplifiers of the type shown by FIG- 9, the output of the first energizing the input of the second through a 2M summing resistor 167, and also connected to the output of the second by means of two equalseries connected resistors of 33K designated 173 and 174. Connecting the series junction of these resistors to ground is a 1.2K resistor 176, and this junction is further connected to the input of the elevation signal summation 'amplifier 24 in the Elevation Channel A by means of a summing resistor 17 2.

Input signals energizing the first summation amplifier 165 are added and amplified and the amplified sum is conducted by line. 168 to output line 52. This signal is also injected to the input of the second summation amplifier 166 through the 2M summing resistor 167 where it further amplified by a gain of one, its polarity inverted, and it is conducted to positive output line 169. Hence, second amplifier 166 provides the means for generating an equal and opposite polarity output signal for the initially summed and amplified signal on line 52. However, if the signal provided by amplifier 166 on line 169 is not equal and opposite to that on line 52, the port and servo amplifiers each energized by one of these lines are driven unequally, wherein the elevons are not difierentially displaced by equal andopposite amounts, and a pitch error results. Compensation for this resulting pitch error is made by feeding an error correction signal to the elevation signal summation amplifier 24 through the above-mentioned summing resistor 172. This error signal is derived from the mid-point of the series connected resistors 173 and 174, when the potentials on lines 169 and 52 are not equal and opposite, however when these signals are equal and opposite the mid-point of the voltage divider series connected resistors 173 and 174 is at zero potential whereby no error correction signal is transmitted.

Summary Summarizing, the autopilot above-described in detail illustrates a preferred embodiment of a system responsive to two commanding stimuli for simultaneously establishing the elevation and azimuth headings of a high speed self-propelled aircraft, to direct this aircraft along a de sired flight path prescribed by the commanding stimuli. Part of this system further responds to any deviation of the aircraft from the thus established headings due to outside forces or inherent aerodynamic instability of the aircraft to stabilize its attitude along this commanded path. Generally, this devicecomprises two continuously operating servo channels adapted to simultaneously position two moveable controller surfaces on the aircraft which aerodynamically determine the flight heading. Each of the servo channels performs independently in positioning both controller surfaces indirecting aircraft heading in a given spatial plane, and jointly "in their superimposed control resulting in aircraft biplanar. direction. The response of each servo slaving channel is modified in a nonlinear manner to compensate for theaerodynamiccharacteristics of the particular type of aircraft utilized, and the characteristics of the command stimuli fromithe intelligence system employed. Afurther predetermined programing system is introduced to-modify one of the commanding stimuli for directing the aircraft heading in a given plane in. accordancenwith a linear or non-linear mathematical function. e

Obviously in performing'the.above-mentioned functions, modification may be made. in the. servo, channels.

and in themanner of their compensation, whether linear or non-linear, to provide for the varying characteristics of alternative. command stimuli generating devices, or varying aircraft aerodynamic characteristics. Furtherchanges in the stimuliprograming system, may bereadily devised inv accordancewith this'teaching, for-example, the programing system for the altitudepathmay bemodified by the eliminationofo'ne of the altitudereferencea or the aircraft may be directed to dive. from high release. altitudes in a series ofgraduated constant dive angles. These and various other changes may: be made by thoseskilled in the art throughout'this device. in accordance with the teachings herein, and accordingly the above-disclosed system is to be considered merely. as a preferred. embodiment of the teaching, while the invention is to belimited only' by features as set forth in the claims.

The invention described herein may be manufactured and used by or for the Government of the United States of Arnerica for governmental purposes without. the payment of any royalties thereon or therefor.

What is claimed is:

1. In an autopilot for an aircraft having two moveable flight path controller surfaces a rfirst servo system comprising an altitude error signal generator and a pitch reference generator, a first signal mixing means responsive to the signals from said generators for algebraically summing and amplifying their values, a first limiting means, the first limiting means adapted to limit the amplified output of the first signal mixing means, a plurality of pitch feedback voltages, a second signal mixing means, the second signal mixing means responsive to the limited output of the first signal mixing means and the plurality of pitch feedback voltages for transmitting an output signal proportional to the amplified algebraic sum of their values, a second limiting means, the second limiting means responsive to the second signal mixing means for limiting its output, two electromechanical positioning devices, each responsive to the output of the second limiting means for varying the aircraft flight controller surfaces, a plurality of feed-back generating means, the feedback generating means responsive to the variation of the aircraft flight controllers in such direction as to produce aircraft pitch for transmitting said pitch feedback voltages; a second servo system comprising an azimuth error signal, a third signal mixing means responsive to this azimuth error signal, a third limiting means responsive to the third mixing means output for limiting its maximum value, a roll reference generator and a plurality of roll feedback voltages, a fourth signal mixing means responsive to the third limited signal mixing means output, the roll reference generator, and the roll feedback voltages for algebraically summing and amplifying these values, the two electromechanical positioning devices simultaneously responsive to the fourth signal mixing means for varying the aircraft flight controllers, the feedback generating means additionally responsive to any variation of the aircraft flight controllers which produces aircraft roll for transmitting said roll feedback voltages.

2. In the device of claim 1, the pitch reference generator, and the roll reference generator comprising a two axis gyro responsive to pitch and roll of the aircraft from selected references for generating pitch and roll reference signals indicative in magnitude and polarity of the amount of deviation and direction of deviation of the aircraft from the reference standards.

3. In the device of claim 1, means for integrating the azimuth error signal, the third signal mixing means responsive to the azimuth error signal and the integrated azimuth error signal for algebraically summing their values.

4. Flight control apparatus for an aircraft having two elevons comprising first reversible means for causing displacement of both elevons in like direction in re sponse to automatically generated elevation command signals, second reversible means for causing diiferential elevon displacement superimposed upon the unison displacement in response to an automatically generated azimuth command signal, a limiting means associated with the first reversible means for limiting the maximum unison elevon displacement, whereby differential elevon displacement takes precedence over unison elevon displacement for selected equally large elevation and azimuth command signals. I

5. In an autopilot for a high speed pilotless aircraft lraving two moveable surface flight path controllers, means for self-directing the elevation flight path in a predetermined manner to a selected low level altitude and stabilizing it about this low level altitude comprising an altimeter, a pitch generator, a compensated alti tude reference generator, signal mixing means,.two surface controller positioning actuators, and'a plurality of,

22 altitude and a reference altitude, said reference genera: tor including means for simulating intermediate reference altitudes while the actual altitude error exceeds predetermined levels, the signal mixing means energized by the altitude reference generator, and the pitch generator to provide a driving signal proportional to the algebraic sum of their values, the two actuators interposed in circuit between the mixing means and the two moveable surface controllers, the plurality of feedback generating means responsive to the unison displacement and direction of displacement of the flight path controllers to generate a plurality of reversible feedback voltages, the signal mixing means responsive to these feedback voltages for diminishing the driven signal accordingly, wherein the flight path controllers are positioned in unison to substantially reduce the driving signal to zero.

6. In an autopilot for controlling the flight path of a high speed pilotless aircraft in azimuth and elevation, servo mechanism means responsive to the altitude of said aircraft to self-direct its elevation path comprising means for generating a signal indicative of aircraft altitude, means responsive to the altitude signal for generating an altitude error signal, and means responsive to the altitude error signal for controlling the aircraft elevation heading, the altitude error generating means comprising limiting means responsive to,the altitude generator for providing constant value signals for all altitudes above a selected first altitude level, an altitude reference generator, signal mixing means, and altituderesponsive switch means, the signal mixing means responsive to the limited altitude generator signal and the altitude reference generator to derive an amplifier error proportional to the difference of thesesignals, the switch means responsive to the altitude generator for all signal values representative of altitudes below a second selected level for varying the amplification provided by the signal mixing means, whereby the altitude error generating means provides three types of error signals depending upon the altitude signals above and below two selected altitude levels, a constant error representing the difference between a fixed first altitude'level andan altitude reference, the second a variable error representing the difference between the present aircraft altitude and an altitude reference, and the third a variable error representing the difference between present aircraft altitude and an altitude reference amplified by a factor greater than that provided by the first and second type errors.

7. An autopilot for controlling in azimuth and elevation a pilotless aircraft of the type having two moveable elevons and fixed stabilizer controllers comprising an elevation channel and an azimuth channel, the elevation channel including in cascaded array an altitude reference compensator for generating an elevation command signal, and a first and second signal mixing means, the azimuth channel including signal mixing means; a vertical reference generator associated with both channels for producing a signal indicative of aircraft pitch from a selected reference attitude, and a second signal indicative of aircraft roll from asecond selected refer ence attitude, the first elevation signal mixing means responsive to the elevation command signal, and the aircraft pitch signal. for generating a difference-signal proportional to the algebraic sum of their values, means responsive tothe displacement of the two elevoncontrollers for generating a plurality of feedback signals, the magnitude of the algebraic sum of which is indicative of the angles of elevon displacement in like direction withrespect to a fixed reference on .the aircraft structure and the resulting polarity of the algebraic sum feedback generating means; thealtitude reference generator being connected to thealtimeter to produce error signals based on the difference between the actualof which is indicative of the direction of unison elevon displacement, the second; elevation signal mixing means responsive to the difference signal from the first elevation mixing means and the plurality of pitch feedback. i I

signals to provide a pitch driving signal proportional to the algebraic sum of these signals, mean si'responsive to the elevon movement for generating a second plurality of feedback signals, the magnitude of the algebraic sum of which is indicative of the absolute angle of differential elevon displacement and the polarity of the algebraic sum of which is indicative of the relative direction of difiFerential elevon displacement; an azimuth command signal, the azimuth channel mixing means responsive to the second plurality of feedback signals, the aircraft roll signal, and the azimuth command signal to generate an azimuth driving signal proportional to the algebraic sum of their values, two reversible control devices, each adapted to position one of the aircraft elevons, the pitch driving signal from the elevation channel energizing both control devices to position the elevon in like direction, and the azimuth driving signal from the azimuth channel simultaneously energizing both control devices to differentially position the elevons.

8. In an autopilot for controlling the flight path of a high speed pilotless aircraft in azimuth and elevation, means responsive to the altitude of said aircraft to control the elevation flight path in a predetermined manner comprising means for generating a signal representative of a selected altitude reference, means responsive to the instant altitude of the aircraft and the selected altitude reference to derive an error signal proportional to the difierence of these signals, means for attenuating this error signal difference, means responsive to said attenuated difference signal to position the aircraft elevation heading to reduce this error to zero, and additional means responsive to all values of aircraft altitude below a preselected altitude point for varying the attenuation means whereby the aircraft elevation positioning means may more rapidly reduce the error difference signal to zero.

9. In an autopilot for controlling the flight path of a high speed aircraft in azimuth and elevation, means responsive to the altitude of said aircraft to control the elevation flight path in a predetermined manner, said means comprising means responsive to all values of altitude above a first selected altitude point for directing the aircraft to assume a constant elevation angle, and responsive to all values of altitude below the first selected point for directing the craft to assume an elevation angle proportional to the altitude, an azimuth command signal, means responsive to the azimuth command signal for controlling the azimuth heading of the aircraft, said azi muth controlling means and said elevation controlling means operative simultaneously and continuously whereby the aircraft is directed along a desired azimuth heading while simultaneously following a preselected elevation path.

10. An altitude comparing and programing device for an autopilot comprising an altitude responsive variable voltage generator, an altitude reference signal generator, two variable value attenuating conductors, a clipper circuit, a signal mixing means, and voltage responsive switching means, the signal generator energizing the input of the signal mixing means through thefirst of the attenuating conductors, the second attenuating conductor electrically intermediate the altitude responsive voltage generator and the signal mixing means input, the clipper circuit responsive to the variable altitude voltage transmitted by the second attenuating conductor to limit the maximum value of signal conducted therethrough, and the voltage responsive switch energized directly by the altitude generator and responsive to all voltages below a selected level for equally varying the attenuation provided by both the attenuating conductors.

References Cited in the file of this patent UNITED STATES PATENTS 2,417,821 Harcurn Mar. 25, 1947 2,507,304 Hofstadter May 9 1950 2,531,458 Nye Nov. 28, 1950 2,558,096 Markusen June 26, 1951 2,579,528 Williams Dec. 25, 1951 2,593,014 Divoll Apr. 15 1952 2,603,433 Nosker July 15, 1952 2,603,434 Merrill July 15, 1952 2,655,328 Frystak Oct. 13, 1953 

